It is well known that the outputs from the cathode terminal for Red, Green and Blue of the color cathode ray tube (CRT) must be well tuned to reach a balance allowing an intermediate Whiteness is obtained when the color signal is absent. Besides, the required proportion of the Red, Green and Blue color to generate the Whiteness must be kept same, regardless of the adjustment of the Brightness control button accessible by the users, in order to have a color temperature independent from the Brightness level. The above function is generally known as color tracking or gray scale tracking which controls the outputs from Red, Green and Blue cathodes to have the same proportion at all Brightness levels ranging from the maximum brightness to minimum brightness.
In recent days, some CRT users in particular areas, e.q. computer-aided graphics, computer-aided desktop publishing, request a very high level of color tracking capability. They, typically, request the color temperature be unchanged when the brightness level or contrast level is adjusted. If the color tracking is not good enough, the raster may look a bit of yellow under one brightness level while a bit of blue under another brightness level.
Typically, the brightness level of the CRT is controlled by the voltage applied on the control grid terminal, also known as G1 terminal, within the CRT. Therefore, the manufactures of the display device employing the CRT typically adopt a narrow adjustable range of the applied voltage, or add compensation circuit in Red, Green and Blue clamping circuit, in order to assure a good color tracking capability.
In light of the above mentioned stringent request for color tracking capability of the CRT, one object of the invention is to provide a method to automatically track the color regardless of the adjustment of the brightness level.